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Remark: a disk with k interior vertices has 2k+n-2 faces (Euler) but only k interior constraints, so each interior vertex adds two degrees of freedom against one constraint -- depth is freedom-positive and Phi can only retain or enlarge below the interior-free floor 2^(n-2). Motivates the lower bound and replaces the prior TODO sketch. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>